Sound forms virtual test tubes
By Eric Smalley, Technology Research NewsMicrofluidics devices that mix and transport tiny amounts of liquid are poised to change biology and chemistry, allowing for more experiments and tests to be carried out using labs-on-a-chip.
These devices usually consist of microscopic chambers and tunnels made from silicon, glass or plastic.
Researchers from the University of Washington have taken a different tack. Their microscale device uses sound waves to trap and mix tiny amounts of substances.
The sound waves create a virtual container that "resembles the eddies that form behind bridge pillars in a river," said Daniel Schwartz, a professor of chemical engineering at the University of Washington. A stable eddy traps water in a circular pattern, preventing it from flowing downstream.
Two recirculating eddies naturally form adjacent to a cylindrical pillar. When the flow direction is repeatedly reversed, four recirculating eddies form.
The researchers used a low-intensity sound wave oscillating at 75 hertz, or cycles per second, to create four microscale recurring eddies around a gold cylinder that measured four fifths of a millimeter in diameter.
The eddies acted as containers capable of trapping microscopic amounts of fluid and particles, according to Schwartz.
This no-walls method is also flexible, Schwartz said. "The size of the trap is controlled simply by changing the sound wave frequency, and the flow speed is controlled by changing the sound wave amplitude," he said. Amplitude is the strength, or loudness, of the sound wave. "Both of these acoustic properties control the degree of chemical trapping within the eddies," said Schwartz.
Chemical mixing is more difficult at the microscale because there is no turbulence, making the action of mixing liquids more like kneeding together two types of dough than swirling cream into coffee.
In microfluidic devices, mixing usually takes place across an interface between fluid streams. "In our approach there are no inlet and outlet streams," said Schwartz. "Fluid walls are spontaneously created and adjustable," he said. The mixing can take place in a stationary spot rather than in fluid streams traveling down channels, which makes it easier to analyze the results, he said.
The ability to turn the eddies on and off allows the researchers to trap and control fluid and particles in place, according to Schwartz.
To observe the flows, the researchers used a pulsed laser that was synchronized to the acoustic frequency.
The researchers demonstrated the system by using the eddies to cause vitamin C to react with various levels of oxygen. "Multiple concentrations [were] prepared within each eddy by simply turning a knob," said Schwartz.
The method could be used to trap and observe single cells, said Schwartz. "As river eddies can trap pop bottles, the microscopic acoustic streaming eddies can trap small objects," he said. "The ability to spontaneously form and dissolve the eddy traps could allow targeted trapping, treatment, observation and release of cells in fluid streams."
The researchers tested their prototype cylinder in a relatively large container. They are now working on embedding their system within a microscale channel, according to Schwartz.
It should be possible to integrate acoustic-based analytical, reaction or cell treatment components into microfluidic devices within a few years, said Schwartz.
Schwartz's research colleagues were Barry R. Lutz and Jian Chen. The work appeared in the April 1, 2003 issue of the Proceedings of the National Academy of Sciences. The research was funded by the National Science Foundation (NSF) and the Center for Process Analytical Chemistry, an Industry/University of Washington Consortium.
Timeline: 3-5 years
Funding: Corporate, Government, University
TRN Categories: Microfluidics and BioMEMS
Story Type: News
Related Elements: Technical paper, "Microfluidics without Microfabrication," Proceedings of the National Academy of Sciences, April 1, 2003.
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May 7/14, 2003
Page One
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